Many battery-powered handhelds such as, for example, mobile phones or electronic notebooks contain complex integrated semiconductor circuits powered by one or more supply voltages. These supply voltages are often generated by voltage regulators, integrated in the semiconductor circuits, from a battery voltage. For this purpose in these devices so-called low dropout voltage regulators are often used which are capable of furnishing a stable regulated voltage even when the difference between the battery voltage and the desired supply voltage is very small. This is why the battery voltage must be only insignificantly higher than the desired output voltage and as a rule the dissipation loss of the voltage regulator is very low. In addition, the voltage regulator is capable of stabilizing the supply voltage even when the battery voltage has been greatly reduced due to discharge.
Voltage regulators may be configured with a simple single-stage feedback loop. Shown in FIG. 1 is a prior art variable voltage regulator as described, for example in the German Semiconductor Circuit Textbook by Tietze and Schenk, published by Springer-Verlag, 12th edition, page 929. The controller in this voltage regulator is formed by a power transistor disposed between the input voltage terminal of the voltage regulator and the supply voltage terminal of a load symbolized in FIG. 1 by the current sink Iout and which is controlled by a feedback signal of an amplifier termed error amplifier in FIG. 1 whose input receives a signal as a function of the supply voltage of the load and which outputs the feedback signal as a function of the difference between the supply voltage and a nominal value. For further stabilization of the supply voltage an output capacitor Cout is usually inserted in parallel with load. The accuracy of the voltage regulator is dictated by the loop gain of the error amplifier which needs to be selected sufficiently high for correspondingly high requirements.
However, this circuit has some drawbacks. For one thing, the feedback circuit becomes unstable at a very low load current Iout in tending to oscillate. The output impedance of the power transistor forms together with the output capacitor Cout a low-pass which in circuit terminology is usually termed a pole position as derived from a mathematical description of the transient response widely used in circuitry by means of the Laplace transformation. In this arrangement the transient function of a low-pass is described by a function comprising a zero position in a polynomial denominator.
A second pole position of the voltage regulator as shown in FIG. 1 is formed by a low-pass consisting of the gate capacitance of the power transistor and the output impedance of the error amplifier. The second pole position normally has a lower frequency than the first pole position. Since, however, the output impedance of the power transistor diminishes with a reduction in the load current, the first pole position tends to drift to an increasingly lower frequency the lower the load current and can thus attain the value of the frequency of the second pole position. This results in the phase of the feedback signal being shifted through 180° and due to this positive feedback the voltage regulator becomes unstable.
Known further in feedback control systems (e.g. in the German textbook thereon by O. Föllinger, published by Hüthig Buch Verlag, 7th edition, page 270) are cascaded feedback loops each of which can be optimized to thus feature improved performance as compared to single-stage feedback loops. Applying this to the present case of the feedback circuit for voltage regulators, this could result in a circuit, for instance, as shown in FIG. 2. With the two-stage feedback circuit as shown in FIG. 2 the drawbacks of the single-stage feedback circuit as described above can be eliminated to a certain extent. This time, the controller is formed by a power transistor whose main current path—which with field-effect transistors is formed by the drain/source channel and in bipolar transistors by the collector/emitter circuit—is disposed between the input voltage terminal Vin and the supply voltage terminal Vout which supplies the load. The outer loop is formed by an error amplifier, the one input of which receives a signal as a function of the supply voltage of the load and whose other input receives a reference voltage and which outputs the feedback signal as a function of the device of the supply voltage from a nominal value. With this feedback signal the non-inverting input of an output amplifier is controlled. The inverting input of the output amplifier is connected to a signal as a function of the supply voltage of the load. The output amplifier thus forms an inner feedback loop capable of working with a lower loop gain than the feedback loop in the single-stage configuration as described above, since the accuracy of the voltage regulator is dictated by the loop gain of the error amplifier.
The bandwidth of the outer loop is defined by a compensating capacitor CC connected to the output of the error amplifier. The compensating capacitor CC forms together with the output impedance of the error amplifier the pole position of the outer feedback loop. As described above, at very low load currents the other pole position of the output amplifier is shifted in the direction of lower frequencies. If the pole positions of the inner and outer loop have the same frequency the feedback circuit becomes unstable. Although this can be counteracted by suitably selecting the capacitor at the output of the error amplifier, this involves very high capacitance values taking up a lot of space on the chip; in other words, there possibly not being enough room to integrate the capacitor in the semiconductor circuit and it thus needs to be applied externally to the chip. This complicates such a feedback circuit and makes it expensive.
Another drawback of this circuit becomes evident when the load element has a very high current requirement, for instance due to the output being short-circuited to ground. In most voltage regulators this is counteracted by an additional circuit for limiting the output current. As soon as a critical maximum permissible current is attained the power transistor is turned off. In the turned off condition the output of the voltage regulator is grounded and the output of the error amplifier increases up to a maximum permissible potential corresponding to its positive operating voltage, for example. Once the short-circuit is eliminated, the voltage at the output of the voltage regulator spikes since the capacitor at the output of the error amplifier first needs to be discharged to allow the input voltage of the output amplifier to fall. This voltage spike may be damaging to the load being supplied.